North Carolina, which self-reported that 13 football players committed secondary NCAA violations by reselling shoes given to them as part of the school's apparel deal, has contacted at least three other schools which it learned might have had players sell shoes to the same retailer.

A source told ESPN that the retailer told UNC it had bought shoes from other schools, including Michigan, Marquette and California. Officials from all three schools acknowledged, through athletic department spokesmen, that they were contacted by North Carolina and are in the process of investigating the claims.

On Monday, North Carolina suspended players found to have participated in the sale of shoes for between two and four games. The suspended players sold Air Jordans, which they were given as part of the university's Jordan sponsorship and which go for thousands of dollars on the resale market.

The exclusive shoe industry is relatively new to the larger college sports market, though it dates back at least 15 years to when Nike co-founder and chairman Phil Knight made special shoes for University of Oregon teams. It exploded five years ago when Oregon football players received pairs of retro Air Jordan 3s and 4s in Ducks colors; they were instantly worth $3,500 on the open market. That same year, two Oregon basketball players, Ben Carter and Dominic Artis, were suspended for selling player exclusives.

ESPN had shoe-marketplace site StockX review its database of about 17,000 different pairs of shoes to see how many player exclusives it had tracked as sold, along with the average price of the sale. StockX also tracks the value of shoes in real time.

Michigan, with 23 pairs, had the most shoes on the exchange. It also had the highest average price of $4,671 a pair.

"Oregon obviously has a long and storied history with Nike, but Michigan is the cool new kid on the block due to the continuing hype surrounding its landmark [Jordan] deal (which began in August 2016)," StockX CEO Josh Luber said.

Michigan football spokesman Dave Ablauf said the number of sales tracked by StockX doesn't mean Wolverines players sold the shoes. Ablauf said executives and celebrities receive the special shoes, as do Nike's Michigan endorsers in the pros. The school itself also might have donated, at times, some pairs to charity.

Sources tell ESPN that school exclusives made for basketball teams are generally limited to 50 pairs, while football-team exclusives are in the 200-300 range.

Ablauf said Michigan players sign a form that acknowledges that selling the shoes would jeopardize their eligibility. The shoes are also marked by the equipment staff with the player's name or uniform number, making it more difficult to sell them anonymously.

Marquette spokesman Mike Broeker said the school now will stipulate that players wear the shoes multiple times, which should decrease their value on the open market and make it less tempting to sell them. Marquette is also stitching player numbers into the shoes.

Oregon now takes the most drastic measures to avoid having athletes sell the exclusive shoes. The team issues them to be worn for travel and certain events, but the equipment staff collects them and stores them immediately afterward in trunks. Players each have space in a trunk and get all their pairs back when their eligibility expires. Redshirt seniors picked up five pairs of player-exclusive Jordans after the football team returned from its bowl game last season.

StockX data reflects that the most valuable "player exclusive" sold was the Air Jordan V Retro Fab Five PE, which went for $11,500. StockX said 13 pairs of Jordan V Michigan player-exclusive retros were also sold, with an average price of $3,126 each.

"Player-exclusive sneakers represent the ultimate status symbol for collectors, with demand from wealthy alumni driving prices to stratospheric levels," said Luber, who noted that player exclusives yield prices that easily fall in the top 1 percent of all collectible-shoe prices.

Oklahoma and Florida have recently joined the Jordan brand and will have exclusive shoes for their athletes soon.